1. Field of the Invention
This invention relates to the field of liquid urea-sulfuric acid compositions, and particularly to such compositions having reduced corrosivity to stainless steel. The compositions are stable homogeneous solutions of urea-sulfuric acid reaction products of significantly reduced corrosivity toward stainless steels even at elevated temperatures and under flow conditions. Thus, they enable the use of such urea-sulfuric acid reaction products at elevated temperatures and/or under flow conditions during their manufacture, transport or use in stainless steel equipment.
2. Description of the Prior Art
Both urea and sulfuric acid are widely used for a variety of purposes in numerous industries as fertilizers, soil adjuvants, chemical treating agents, chemical precursors and reactants. They are sometimes useful in combination, particularly in the agricultural industry, when the simultaneous addition of urea and sulfur to the soil is desired.
Previous investigators have observed that urea, sulfuric acid and, optionally water, can be reacted to form concentrated solutions of urea and sulfuric acid reaction products in which the urea is present as mono- and/or diurea sulfate. However, they did not recognize that the reaction product solutions of different urea/sulfuric acid ratio exhibit markedly different corrosivity towards stainless steel particularly at elevated temperatures and/or under flow conditions, or that the corrosivity of all of the solutions towards stainless steel could be markedly reduced by the addition of corrosion inhibiting amounts of cupric ion-containing compounds.
D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914).
L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. (50.degree. F.) and 25.degree. C. (77.degree. F.). "Ternary Systems of Urea and Acids. I. Urea, Nitric Acid and water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS, 56, 549-53 (1934).
In the article "Adding Plant Nutrient Sulfur to Fertilizer," Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers.
Jones, U.S. Pat. No. 4,116,664 disclosed what is referred to therein as a tortuous, multistage process for producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 200.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred and that the products can be used as fertilizers.
A wide variety of compounds, including cupric sulfate and dialkylthioureas, are known to reduce the corrosivity of sulfuric acid to stainless steels and carbon steels.
Although these investigators disclosed several characteristics of urea-sulfuric acid combinations and methods of making those combinations, and that the products were useful soil adjuvants and/or fertilizers, they did not recognize that the corrosivity of the urea-sulfuric acid reaction products in either concentrated or diluent form to stainless steel varies dramatically as a function of the urea/sulfuric acid weight ratio, or that the corrosivity of all solutions to stainless steel at elevated temperatures and/or under fluid flow conditions can be dramatically reduced by the addition of corrosion inhibiting amounts of cupric ion-containing compounds.
Previous investigators also were not aware that the corrosion characteristics of the urea-sulfuric acid compositions of this invention differ markedly from those of sulfuric acid. For instance, all of the sulfuric acid inhibitors, with the exception of cupric ion, have little or no beneficial effect on the urea-sulfuric acid compositions, and may significantly increase corrosion rate. Furthermore, none of the known inhibitors have any significant beneficial effect on carbon steel corrosion by compositions having urea/sulfuric acid molar ratios between 1 and 2. Carbon steel corrosivity is relatively low within that composition range but is intolerably high with compositions having urea/sulfuric acid molar ratios above 2 or below 1. Thus, carbon steel corrosivity increases dramatically as sulfuric acid concentration is either increased or decreased relative to urea concentration outside this range of molar ratios. Another anomalous characteristic of these compositions is that some, but not all of them are significantly more corrosive to stainless steel than they are to carbon steel. Thus, 10-0-0-19 corrodes AISI C-1010 carbon steel at a rate of 56 mils per year under static conditions at 150.degree. F. and corrodes AISI type 316 stainless steel at 300 mils per year under identical conditions. These properties are not characteristic of sulfuric acid.
It is therefore one object of this invention to provide improved, liquid urea-sulfuric acid compositions.
It is another object of this invention to provide liquid urea-sulfuric acid reaction product compositions which are noncorrosive to stainless steel even at elevated temperatures or under fluid flow conditions.
Another object is the provision of urea-sulfuric acid compositions of reduced corrosivity to both stainless steel and carbon steel.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings and the appended claims.